For the source text click/tap here: Yevamot 39
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In earlier periods when the yavam and yevamah had pure intentions, and performed yibum with the intention of fulfilling the mitzvah, yibum was preferable.
In later years, when people could no longer be relied upon to have the proper intent, chalitzah became the preferred option.
This most basic question is discussed on our daf, where we find a disagreement between Abba Shaul and the Chachamim.
Abba Shaul argues that a person who performs yibum because he finds the yevamah beautiful, because he wants to be married to her or for some other reason (the Rivan suggests that this refers to monetary gain), it is as though he engaged in a forbidden sexual act.
The Hakhamim point to the passage (Devarim 25:5) that commands the yavam to “come upon” her, and see no qualifications in the performance of the mitzvah.
We explore the notion of intent and heterodox opinions regarding yibbum and Chalitzah and which is preferable nowadays.